System and method of channel performance monitoring

ABSTRACT

The channel impulse response of a communication channel is determined from measurement of the cross correlation between the channel output signal and a delayed locally generated pseudorandom noise sequence signal. The undelayed pseudorandom noise sequence signal is utilized as a probe signal and is added to any information signal prior to being supplied to the communication channel being monitored. The output of the channel is hard limited and polarity-coincidence compared to the timedelayed pseudorandom noise sequence signal.

OQ'i-Qw'i'i Xi? 3969 w +3 United States Patent 1 3,69 ,6 3 Smith [451Sept. 26, 1972 4] SYSTEM AND METHOD OF CHANNEL OTHER PUBLICATIONSPERFORMANCE MONITORING Briggs et a1. Correlation analysis of Processdynamics [72] Invenmr' Schenectady Proc. Inst. of Mech. Eng. 1964- 1965Vol. 179 Pt 3H,

[73] Assignee: General Electric Company T1 212 e68 p. 37/51.

. Truxall (Book) Aut. Feedback Control Systems Me- [22] sept- Graw Hill1955 TJ214- T7 pages 43 7- 439. [21] Appl. No.: 76,829 Hewlett- PackardJournal Nov. 1969 Primary Examiner-Felix D. Gruber [52] U.S.Cl..235/l8l, 178/69 A, 235/1503, Attorney pau] A. p k John R Ahem Julius J324/57 324/77 H, 325/67 Zaskalicky, Louis A. Moucha, Frank L. Neuhauser,

[51] Int. Cl. ..G0lr 31/28, G06f 15/34 Oscar B W dd ll a d Jo e h BForman [58] Field of Search...235/181, 150.3, 150.31, 150.5,

235/1506, 150.53; 324/57 R, 57 PS, 77 G, [57] ABSTRACT 77 H; 178/69 A;325/426567 The channel impulse response of a communication channel isdetermined from measurement of the cross correlation between the channeloutput signal and a [56] References Cited delayed locally generatedpseudorandom noise sequence signal. The undelayed pseudorandom noiseUNITED STATES PATENTS sequence signal is utilized as a probe signal andis 3 249 911 5/1966 Gustaf-SS0 235/181 x added to any information signalprior to being sup- 3350644 10/1967 McNair 325/65 x plied to thecommunication channel being monitored. 3430l49 2/l969 William's "5 X Theoutput of the channel is hard limited and polarity- 3,517,175 6/1970Williams ..235/18l x 332 12?2 32;253:132

1 1 Claims, 4 Drawing Figures 5 Ma) ,/4 camera/idem med pm m) Hard M2 333,3333 De/ay maflfle/ Amy/{er zwa 1, 42, l 4

l/ar/a pm- 7') (7064 D/dy d=/7J A reememsg- 0/54 meme/7 s v/' Var/Zzb/ey f 5/0/04 PQl/IffZb/e e a C0 44 6/)? er Var/'a'b/e f/ock De/ag 0 kf 4 9Agreement: Disagreement:

SYSTEM AND METHOD OF CHANNEL PERFORMANCE MONITORING My invention relatesto system and a method for determining the impulse response of acommunication channel, and in particular, to a system and method using apseudorandom noise sequence as a probe signal transmitted through a widebandwidth channel simultaneously with an information signal anddetermining the channel impulse response from measurements employingpolarity coincidence correlation techniques.

The performance of a communication channel over a particular frequencybandwidth is required to be known in determining the capability of suchchannel for transmitting information with distortion not exceedingprescribed limits. Thus, the performance or transmission characteristicsof the channel limits the effective bandwidth of such channel. Ameasurement or determination of the impulse response of thecommunication channel is a most convenient means for monitoring channelperformance. The channel impulse response may be defined as the timeresponse of the channel to a unit impulse function, any deviation of thetime response at the output of the channel from the unit impulsefunction being due to amplitude and phase distortion which ischaracteristic of the particular channel being monitored.

Several techniques are available for channel performance monitoring,that is, determining the channel impulse response. One particulartechnique for channel monitoring, an improved version being the subjectof my invention, is the use of a pseudorandom noise sequence as a probesignal which is transmitted through the channel simultaneously with anyinformation or data signal. Cross correlation analysis between thecombined information and pseudorandom signal at the channel output andthe probe signal yields the channel impulse response as described in anarticle by J .D. Balcomb, HB. Demuth and BF. Gyftopoulos, A CrossCorrelation Method for Measuring the impulse Response of ReactorSystems, Nuclear Science and Engineering, Vol. 11, 1961, pps. 159-166.In another article by CR. Abbey, Data Channel Monitoring by CorrelationTechniques, IEEE Transactions on Aerospace and Electronic Systems,January 1968, pps. 58-64, such technique is also described, in bothcases the correlator being of an analog type, that is, employing ananalog multiplier and integrator. The analog type correlator issatisfactory for narrow channel bandwidth applications and where thedistortion to be measured is relatively large, but in the case of a widechannel bandwidth, small distortion application, which is the subject ofmy invention, the analog correlator technique is not satisfactory sincethe accuracy of multiplication linearity is limited and the analogintegrator may introduce excessive drift during the integration timerequired by such analog or direct correlation technique.

An alternative to direct correlation is polarity coincidencecorrelation. An article by L.E. Zegers, Common Bandwidth Transmission ofinformation Signals and Pseudonoise Synchronization Waveforms, IEEETransactions on Communication Technolog Vol. COM-J6, No. 6, December1968, pages 796-807, describes the polarity coincidence correlationtechnique in a synchronization system not adapted to yield the channelimpulse response which is the object of applicants invention. Thechannel bandwidth in the Zegers article is also much more narrow thanthe requirement for applicants channel to be described hereinafter.Finally, a US. Pat., No. 3,404,261 to P.G.A. Jespers et al. is alsobased on the polarity coincidence correlation technique but is a muchmore complex system than applicants invention and requires a variablereference signal whose slope is random, and the functions to becorrelated are restricted to amplitude range limits within the range ofthe variable reference level.

Therefore, one of the principal objects of my invention is to provide anew system and method for monitoring the performance of a widebandwidth, small distortion level communication channel.

Another object of my invention is a system and method for determiningthe impulse response of the communication channel by employing polaritycoincidence correlation.

A still further object of my invention is to provide a system and methodfor determining the impulse response during the simultaneoustransmission of other signals, such as information signals, through thechannel.

Briefly stated, my invention is a channel performance monitoring systemwhich uses a pseudorandom noise sequence as a probe signal which may betransmitted simultaneously with an information signal through thechannel being monitored, the circuitry at the output of the channelbeing adapted for measurement of the cross correlation between thechannel output signal and a delayed locally generated pseudorandom noisesequence signal by means of polarity coincidence correlation. The outputof the channel is hard limited and compared to the time-delayedpseudorandom noise sequence signal in a modulo 2 adder which indicateswhen the bit levels or polarity of the two input signals agree(coincide) or disagree (differ). The output of the modulo 2 adder isconnected to a reversible counter which counts the difference betweenthe total number of agreements and disagreements in the bit levels orpolarity of the modulo 2 adder input signals from which the crosscorrelation between the signal at the output of the channel and thetime-delayed pseudorandom noise sequence is determined. The impulseresponse of the channel may then be determined from the crosscorrelations measured for various time delays in the pseudorandom noisesequence.

The features of my invention which I desire to protect herein arepointed out with particularity in the appended claims. The inventionitself, however, both as to its organization and method of operation,together with further objects and advantages thereof may best beunderstood by reference to the following description taken in connectionwith the accompanying drawings wherein like parts in each of the severalfigures are identified by the same reference character and wherein:

FIG. 1 is a block diagram of a first embodiment of my invention;

FIG. 2 is a block diagram of a second embodiment of my invention whereinthe cross correlation function is not dependent on distribution of theinformation signal as in the FIG. 1 embodiment;

FIG. 3 is a block diagram of a third embodiment of my invention whereinonly a single hard limiter is employed and is provided with a variablebias voltage for eliminating the dependency of the cross correlationfunction on the distribution of the information signal, and

FIG. 4 is a block diagram of a system utilized for testing the FIG. 1system.

Referring now in particular to FlG. 1, there is shown a system includinga pseudorandom noise sequence generator which generates a pseudonoiseprobe signal P(t). Probe signal P(t) is a pulse signal wherein eachpulse leading edge coincides with a clock pulse generated in circuit 10,but the period is a random integral number of the clock pulse period.Probe signal P(t) and signal S(t), which may represent an information ordata signal (in digital or analog form) and any noise superimposedthereon, (it being assumed that both the data signal and noise areuncorrelated with the probe signal) are combined in any conventionalelectrical voltage signal summing device 11 such as a twoinput summingoperational amplifier. The summed signal X(t) is the input to acommunication channel 12 whose performance is to be monitored by thedetermination of the impulse response h(t) thereof. The received oroutput signal of channel 12 is designated Y(t) and may be expressed as:

The cross correlation between the received signal Y(t) and the probesignal P(t) is:

lim

2T fY(t)P(z 1m (2) where 7 is the delay between the received signal Y(t)and a locally generated probe signal which may be generated by a secondpseudorandom noise sequence generator 13. As indicated in theabove-identified Abbey article, the substitution of equation (1) and(2), under the assumption that the data signal S( t) is uncorrelatedwith the probe signal P(t), yields:

vp(' (4) An n stage shift register with feedback can produce a maximallength linear binary sequence (pseudonoise sequence) which is repetitivewith length 2" 1 bits and exhibits a quasi-impulse autocorrelationfunction. The normalized autocorrelation function of such a sequence forr T,,, where T, is the period of the sequence, is:

where 8 is the duration of one bit. The autocorrelation function Rpp(7')is repetitive with period T, and in order to achieve the result ofequation (4), the autocorrelation function must be a reasonably goodapproximation to a unit impulse function. This is achieved when thepulse duration 8 is sufficiently small, that is, the pulse duration of abit in the pseudorandom noise sequence P(t) is less than the reciprocalof the channel bandwidth, the sequence period T, must be sufficientlylong to prevent appreciable overlap of the periodically repeated channelimpulseresponses, and the amplitude (2 -1) is small compared toamplitude variations which are to be detected, where n is the number ofbits in the sequence. Under these latter conditions, the crosscorrelation Ryp('T) is proportional to the impulse response of thechannel and may be expressed as:

M yp( (6) where Ryp(0) is dependent on the probability distributionfunction of data signal S(l).

Pseudorandom noise sequence generators l0 and 13 may therefore beconventional shift registers with feedback, the time delay 7 for thelocally generated pseudorandom noise sequence signal P(tr) beingprovided by the clock frequency pulses entering the shift register at adifferent stage from that of generator 10 or taking the output of theshift register at a different stage. Alternatively, generators 10 and 13may be identical (or just one generator employed), and a variable timedelay network is connected to the output of generator 13.

Since one of the advantages of my invention is the ability to monitorthe channel performance while the probe signal P(t) is being transmittedsimultaneously with the data signal S(t), the effect of thesignal-toprobe ratio on the measurement of the cross correlationfunction must be considered. As such ratio increases, the crosscorrelation function Ryp(7) becomes increasingly difficult to measureusing direct correlation techniques since the accuracy of measurement isdependent on analog multiplication linearity and integrator driftlimitations during the required processing time. The alternative todirect correlation, which I employ in my invention, is polaritycoincidence correlation. Polarity coincidence correlation isaccomplished by hard limiting the communication channel 12 output signalY(t) and comparing the polarity (or bit level) thereof with the polarity(or bit level) of the locally generated probe signal P(t-r). Thus, theoutput of channel 12 is connected to the input of a hard limiter circuit14 which develops a two-level signal Z(t) at its output as a function ofthe relative magnitudes of the channel signal Y(t). Channel signals ofvoltage mag nitude greater and smaller than a predetermined valueestablished by a bias voltage B in the hard limiter circuit l4 developthe higher and lower voltage level signals Z(t), respectively. Limitercircuit 14 output signal Z(t) is supplied as a first input to a modulo 2adder (exclusive OR logic circuit) 15. The signal at the output of hardlimiter 14 is, as stated above, a two-level or may be a bipolaritysignal and these two levels (or polarities) are compared on a (clock)bit-by-bit basis with the two levels (or polarities) of delayed probesignal P(t1') supplied as a second input to the modulo 2 adder 15. Theoutput Q(t) of modulo 2 adder yields one level when the levels orpolarities of the two input signals Z(t) and P(tr) agree, and anotherlevel when the input levels or polarities disagree. The twolevel signalQ(t) may be of the same voltage polarity or may be a positive voltagewhen the polarities (or levels) of the two input signals agree and anegative voltage when the polarities (or levels) disagree. The output ofmodulo 2 adder 15 is supplied to the input of a conventional reversiblecounter 16 such that the count output thereof is the difference betweenthe total number of agreements and the total number of disagreements inthe bit levels (or polarities) of the two input signals to the modulo 2adder. v

The cross correlation Ryp(T) between the signals Y(t) and P(t*r) can beexpressed as:

RYP(

number of agreementsnumber of disagreements number of agreements+numberof disagreements V, v. V (7) Since the sum of the total number ofagreements and the total number of disagreements is the total number ofbits compared, the output of the reversible counter 16 divided by thetotal number of bits transmitted is the cross correlation Ryp('r).

The factor previously limiting the use of pseudorandom sequences tomonitor wideband data channel i.e., bandwidth greater than 1 MHz) towithin a desired accuracy corresponding to a very small distortion levelis the lack of a suitable analog correlator, as stated above. My digital(polarity coincidence) correlator, which includes elements 13, 14, 15,16 eliminates the problems of insufficient linearity and driftassociated with the analog multiplier and integrator in the analogcorrelator. However, the digital correlator in FIG. 1 has thedisadvantage, noted in equation (6), of the amplitude of the measuredcorrelation function being dependent on the distribution of thetransmitted data signal S(t), thereby introducing errors during thetransmission of arbitrarily varying data signals.

FIG. 2 illustrates a second embodiment of my channel performancemonitoring system, and in particular, a system wherein the crosscorrelation function R br) is independent of the probabilitydistribution of the data signal S(t) as distinguished from the FIG. 1embodiment. The system in the FIG. 2 embodiment utilizes an array of 2N1 digital correlators of the type illustrated in P16. 1 and utilizesdifferent fixed bias levels (0, 2, 2, 2N, 2N) volts, as one example, forthe hard limiters 14a through 141: and Mn which each have output levelsof i I volt in such exemplary circuit. Thus, a bias of +2 volts meansthat whenever the input signal Y(t) is greater than +2 volts, the outputis at the upper (+1 volt) level and when less than +2 volts, the outputis at the lower (1 volt) level. N is the number of correlators added toobtain measurements at voltage levels greater than zero. N additionalcorrelators are utilized to obtain measurements at levels less thanzero. These correlators plus the one that measures at the zero levelaccounts for the total of 2N I, and specifying N in this manner allowsthe summation of equation (9) to go from N to +N. When the pseudorandomnoise sequences are correlated, the cross correlation of thisconfiguration becomes yp( yp( (a) where where F;(s) is the probabilitydistribution function. As N increases, the effect of the data signalbecomes less significant and for N sufficiently large:

Each digital correlator in my FIG. 2 embodiment has the input thereofsupplied from the output Y(t) of the communication channel 12 andincludes a hard limiter, a modulo 2 adder supplied with inputs from thelimiter and a common local pseudorandom sequence generator 13, and areversible counter. The outputs of the 2N l counters are supplied to theinput of a digital summer 20 which provides the difference between thetotal number of agreements and total number of disagreements signal R('r) as does the reversible counter 16 in the FIG. 1 embodiment. Afurther advantage of my FIG. 2 embodiment is that it reduces the totalmeasurement time for processing the several Ryp(r) measurements. Thisshortening of the total measurement time is due to the fact that whenthe amplitude of the probe signal is small compared to the data signal(a desirable and the general condition), the only contribution to thecorrelation measurement occurs for signal levels close to the decisionlevel of the hard limiter. The introduction of a number of decisionlevels allows correlation measurements to be made over a largeramplitude range thereby reducing the time required for a given accuracy.

FIG. 3 illustrates a third embodiment of my channel performancemonitoring system and a second means for eliminating the dependency ofthe cross correlation function on the probability distribution functionof the data signal S(t). The hard limiter 14 in the digital correlatorhas a bias voltage level B supplied thereto that is automatically variedin predetermined steps, the limiter bias level being determined by thereceived signal level Y(t). The limiter bias level signal 8 is variablein steps of 2m (volts) where m is an integer as in equation (9) andcovers the range N to +N which corresponds to the range from the minimumvalue of %[Y(t) P,(t)] to the maximum value, and this has the effect ofreplacing the array of 2N +1 limiters in the FIG. 2 embodiment with asingle limiter thereby requiring only a single modulo 2 adder l5 andsingle reversible counter 16. The step variable bias voltage signal B isobtained by cancelling the effect of the pseudorandom noise sequencesignal P(t) in the received signal Y(t) at the output of channel 12.Local pseudorandom noise sequence generator 13 provides the identicaloutput signal P(t) as generator (i.e., no delay 1- as in FIGS. 1 and 2)and is connected to the input of a communication channel model 12ahaving identical impulse response characteristics h(t) as communicationchannel 12. The signal at the output of channel model 12a is designatedP (t). Signals Y(t) and P,(t) are subtracted in a conventionaldifferential amplifier circuit 30, and the output signal thereof Y(t) P(t) is supplied to a conventional quantizing circuit 31 which functionsto replace the continuous input signal Y(t) P,(t) with a discretesignal. During any specified time interval, the discrete signal occupiesone of several specified output voltage levels B as determined by thevoltage level of the input signal. The step variable bias signal B atthe output of circuit 31 is supplied to hard limiter circuit 14. Thereceived signal Y(t) is also supplied to the hard limiter as in theFIGS. 1 and 2 embodiment. The output of the pseudorandom noise sequencegenerator 13 is also supplied to a variable delay circuit 32 whichprovides the delayed pseudorandom noise sequence signal P(t 'r) as inthe case of the output of generator 13 in the FIGS. 1 and.2 embodiments.Module 2 adder circuit compares the signal level (or polarity) of theoutput of hard limiter l4 and variable delay circuit 32 on a bit-by-bitbasis as in the case of the FIG. 1 embodiment, and the output of modulo2 adder 15 is supplied to the reversible counter 16 which measures thedifference between the total number of agreements and total number ofdisagreements in the bit levels of the two input signals of the modulo 2adder circuit 15. The FIG. 3 embodiment thus has the advantage of theFIG. 2 embodiment in that the cross correlation function Ryp('r) is notdependent on the probability distribution of data signal S(t) and thefurther advantages of using fewer components than the FIG. 2 embodiment.

FIG. 4 illustrates an experimental embodiment of my channel monitoringsystem for verifying the analytical results described by equations (6)and (7). The system is essentially that of FIG. 1 but utilizing only asingle pseudorandom noise sequence generator 10. The delayedpseudorandom noise sequence signal P(t-*r) is obtained by passing thegenerator 10 output signal through a variable delay network 40. Theoutput of generator 10 is also passed through a fixed delay network 41.Delay networks 40 and 41 are each adapted to effect time delays 1,, ofan integral number of clock periods (n 8). For the particularexperiments described hereinafter, fixed delay network 41 was adjustedto provide a fixed time delay of clock periods and vari able time delaynetwork 40 was adapted to provide time delays up to 40 clock periodssuch that time delay 1- could vary from 20 to +20 clock periods. Itshould be understood that the digital clock 42 is an inherent part ofthe pseudorandom sequence generator and is also utilized for triggeringthe counter, but is not illustrated in FIGS. 1, 2, 3 for purposes ofsimplification. A first variable clock delay circuit 43 is connectedbetween the output of clock 42 and a second input to variable delaycircuit 40 for delaying the clock pulse thereto a fraction of one clockperiod 0 t 8 to compensate for the time delay of the signals passingthrough summer 11 and hard limiter 14 but excluding the time delay ofchannel 12. A second variable clock delay circuit 44 is connectedbetween the output of clock 42 and the triggering input of reversiblecounter 16 and is adapted to provide a time delay of a fractional clockperiod for purposes of centering the clock pulses on the modulo 2 adderoutput signal Q(t). The output of variable clock delay circuit 44 isalso supplied to a second counter 45 which provides at the outputthereof the total number of clock periods, i.e., the sum of the totalnumber of agreements and total number of dis agreements. Thus, theoutput of reversible counter 15 divided by the output of counter 45determines the cross correlation Ryp(T)- In the experiments, correlationmeasurements were successfully made for signal-to-probe ratios in arange from 1 to and prove that the system is capable of measuring thechannel impulse response when an interfering signal, data signal S(t),is present. The results of these measurements also showed a satisfactoryagreement between the measured and calculated channel impulse response.From the above experimental measurements and additional data, it hasbeen determined that the channel performance monitoring systemsdisclosed in the FIGS. 1, 2 and 3 embodiments are capable of monitoringa communication channel having a bandwidth limited only by availablelogic circuitry. Current logic circuitry should allow monitoring ofchannels of up to 50 MHz bandwidth and for detecting very smalldistortions (amplitude variation as small as 0.l dB and 0.5 deviationfrom linear phase over the bandwidth). The cross correlationmeasurements can be made with a probe signal level as low as oneone-hundredth of the data signal amplitude to thereby minimizeinterference with the data signal transmission to a negligible amount.The observation time for a single cross correlation measurement Ryp(T)is a function of the required accuracy (minimum distortion to bedetected) and the total observation time for all the measurements isalso a function of the channel characteristics. The analog correlatorapproach described in the Abbey article and the other prior art systemsdescribed hereinabove are not capable of meeting such strictrequirements nor are they adapted for monitoring such wide band datachannels in real time. The FIGS. 2 and 3 embodiments reduce measurementtime over conventional analog or digital correlator channel monitors,and allow measurements having the accuracy of the FIG. 1 embodiment.Although the FIG. 1 embodiment does not reduce measurement time over theprior art, it does allow measurements to be made over longer times inorder to increase measurement accuracy.

From the foregoing description, it can be appreciated that my inventionmakes available a new system and method for monitoring the performanceof a communication channel which is especially adapted for widebandwidth channels having very small distortion requirements, althoughit is obviously also useful with more narrow bandwidth channels and lessstrict distortion requirements. Having described three particularembodiments of my invention, the intended scope of my invention isdefined by the following claims.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. A communication channel performance monitoring system for determiningthe impulse response of a communication channel having an input andoutput by employing polarity coincidence correlation for obtainingmeasurement of cross correlation between the channel output signal and adelayed locally generated pseudorandom noise sequence probe signal andcomprising first generating means in communication with an input to acommunication channel being monitored for generating clock pulses and afirst twolevel pseudorandom noise sequence probe pulse signal inresponse to the clock pulses wherein the leading edges of the probesignal pulses coincide with the clock pulses and the periods of theprobe signal pulses are random integral numbers of a clock pulse period,summing means connected between an output of said first generating meansand the input to the communication channel for summing the probe signaland a data signal simultaneously transmitted through the channel, theprobe signal being uncorrelated with the data signal and having avoltage level as low as one and one-hundredth of the amplitude of thedata signal to thereby minimize interference with the data signaltransmission to a negligible amount, signal limiting means incommunication with an output of the communication channel for developinga two-level signal being a function of the relative magnitudes of thechannel output signal, second generating means for generating clockpulses and a second probe signal in response thereto corresponding tothe first probe signal but with a controllably variable time delay 1-,means connected to the output of said signal limiting means and saidsecond generating means for comparing the signal outputs thereof on abit-by-bit basis with respect to the clock pulses, the output signal ofsaid comparing means being at a first voltage level when the levels ofthe delayed probe signal and an output of said signal limiting meansagree and being at a second voltage level when the levels disagree, andreversible counter means connected to an output of said comparing meansfor establishing a count which is the difference between the totalnumber of agreements and total number of disagreements at the output ofsaid comparing means over a particular time interval and from which ameasurement of the cross correlation between the signal at the output ofthe communication channel and the probe signal may be determined. 2. Thecommunication channel performance monitoring system set forth in claim 1wherein said summing means is a two-input electronic operationalamplifier, said signal limiting means, said comparing means and saidreversible counter means forming a digital correlator to thereby adaptthe system for wide bandwidth monitoring and small distortion leveldetection. 3. The communication channel performance monitoring systemset forth in claim 1 wherein said comparing means is a modulo 2 addercircuit. 4. The communication channel performance monitoring system setforth in claim 1 wherein said signal limiting means is a hard limiterelectronic circuit which develops higher and lower voltage levels of thetwo-level signal in response to channel output signals being ofamplitude greater and smaller, respectively, than a predetermineddecision value established by a bias voltage in the hard limitercircuit.

5. The communication channel performance monitoring system set forth inclaim 1 wherein said signal limiting means, said comparing means andsaid reversible counter means are each only single components.

6. The communication channel performance monitoring system set forth inclaim 1 wherein said signal limiting means, said comparing means andsaid reversible counter means forming digital correlator means, saidsignal limiting means, said comparing means and said reversible countermeans each consisting of 2N 1 components connected in an array of a 2N lplurality of parallel connected digital correlators, the output of thecommunication channel connected to inputs of the 2N 1 signal limitingmeans components, the output of said second generating means connectedto inputs of the 2N l comparing means components, and further comprisingdigital summing means having inputs thereof connected to outputs of thereversible counter means components for providing at the output of thedigital summing means the difference between the total number ofagreements and the total number of disagreements over the particulartime interval, the plurality of digital correlators reducing themeasurement time required for a desired accuracy of channel distortionmeasurement. 7. The communication channel performance monitoring systemset forth in claim 6 wherein the 2N 1 signal limiting means componentsare provided with 2N 1 different bias voltages determining 2N 1predetermined decision values of the channel output signal magnitudesabout which 2N l two-level signals are developed.

8. The communication channel performance monitoring system set forth inclaim 1 and further comprising means for automatically varying a biasvoltage level in said signal limiting means in predetermined steps as afunction of the channel output signal to thereby determine a pluralityof predetermined decision values of the channel output signal magnitudeabout which the signal limiting means twolevel output signal isdeveloped, the plurality of decision levels reducing the measurementtime for a desired accuracy of channel distortion measurement.

9. The communication channel performance monitoring system set forth inclaim 1 and further comprising counter means in communication with saidsecond generating means for establishing a count which is the sum of thetotal number of agreements and total number of disagreements and isequal to the number of clock pulses generated over the particular timeinterval, the cross correlation being determined by the countestablished by said reversible counter means divided by the countestablished by said counter means.

10. A method for monitoring the performance of a communication channelby determining the impulse response thereof employing polaritycoincidence correlation for obtaining measurement of cross correlationbetween the channel output signal and a delayed locally generatedpseudorandom noise sequence probe signal and comprising the steps ofgenerating clock pulses and a first pseudorandom noise sequence voltageprobe pulse signal in response to the clock pulses wherein the leadingedges of the probe signal pulses coincide with the clock pulses and theperiods of the probe signal pulses are random integral numbers of aclock pulse period,

passing the first probe signal through the communication channel beingmonitored,

limiting the signal output of the communication channel by developing atwo-level signal as a function of the relative magnitudes of the channeloutput signal,

generating a second probe signal corresponding to the first pseudorandomnoise sequence signal but with a first variable time delay,

comparing the delayed second probe signal and the two-level limitedsignal on a bit-by-bit basis with respect to the clock pulses, thesignal comparison developing an output signal at a first voltage levelwhen the levels of the delayed second probe signal and limited sig'nalagree and at a second voltage level when the levels disagree,

generating a count which is the difference between the total number ofagreements and total number of disagreements of the compared delayedprobe signal and limited signal over a particular time interval for afirst selected delay in the second probe signal, and

generating a count which is the sum of the total number of agreementsand total number of disagreements and is equal to the number of clockpulses generated over the particular time interval to thereby determinea first measurement of the cross correlation between the signal at theoutput of the communication channel and the pulse signal by dividing thedifference count by the sum count.

11. The method for monitoring the performance of a communication channelset forth in claim 10 and further comprising repeating the steps ofgenerating a first probe signal,

limiting the signal at the output of the communication channel,generating a second probe signal with a variable time delay, comparingthe delayed probe signal and limited signal and generating counts whichare respectively the difference between and sum of the agreements anddisagreements over a particular time interval for a plurality ofdifferent variable time delays of the second probe signal including zerodelay for determining a plurality of different measurements of the crosscorrelation between the signal at the output of the communicationchannel and the probe signal from which the channel impulse response isdetermined.

* l II

1. A communication channel performance monitoring system for determiningthe impulse response of a communication channel having an input andoutput by employing polarity coincidence correlation for obtainingmeasurement of cross correlation between the channel output signal and adelayed locally generated pseudorandom noise sequence probe signal andcomprising first generating means in communication with an input to acommunication channel being monitored for generating clock pulses and afirst two-level pseudorandom noise sequence probe pulse signal inresponse to the clock pulses wherein the leading edges of the probesignal pulses coincide with the clock pulses and the periods of theprobe signal pulses are random integral numbers of a clock pulse period,summing means connected between an output of said first generating meansand the input to the communication channel for summing the probe signaland a data signal simultaneously transmitted through the channel, theprobe signal being uncorrelated with the data signal and having avoltage level as low as one and one-hundredth of the amplitude of thedata signal to thereby minimize interference with the data signaltransmission to a negligible amount, signal limiting means incommunication with an output of the communication channel for developinga two-level signal being a function of the relative magnitudes of thechannel output signal, second generating means for generating clockpulses and a second probe signal in response thereto corresponding tothe first probe signal but with a controllably variable time delay Tau ,means connected to the output of said signal limiting means and saidsecond generating means for comparing the signal outputs thereof on abit-by-bit basis with respect to the clock pulses, the output signal ofsaid comparing means being at a first voltage level when the levels ofthe delayed probe signal and an output of said signal limiting meansagree and being at a second voltage level when the levels disagree, andreversible counter means connected to an output of said comparing meansfor establishing a count which is the difference between the totalnumber of agreements and total number of disagreements at the output ofsaid comparing means over a particular time interval and from which ameasurement of the cross correlation between the signal at the output ofthe communication channel and the probe signal may be determined.
 2. Thecommunication channel performance monitoring system set forth in claim 1wherein said summing means is a two-input electronic operationalamplifier, said signal limiting means, said comparing means and saidreversible counter means forming a digital correlator to thereby adaptthe system for wide bandwidth monitoring and small distortion leveldetection.
 3. The communication channel performance monitoring systemset forth in claim 1 wherein said comparing means is a modulo 2 addercircuit.
 4. The communication channel performance monitoring system setforth in claim 1 wherein said signal limiting means is a hard limiterelectronic circuit which develops higher and lower voltage levels of thetwo-level signal in response to channel output signals being ofamplitude greater and smaller, respectively, than a predetermineddecision value established by a bias voltage in the hard limitercircuit.
 5. The communication channel performance monitoring system setforth in claim 1 wherein said signal limiting means, said comparingmeans and said reversible counter means are each only single components.6. The communication channel pErformance monitoring system set forth inclaim 1 wherein said signal limiting means, said comparing means andsaid reversible counter means forming digital correlator means, saidsignal limiting means, said comparing means and said reversible countermeans each consisting of 2N + 1 components connected in an array of a2N + 1 plurality of parallel connected digital correlators, the outputof the communication channel connected to inputs of the 2N + 1 signallimiting means components, the output of said second generating meansconnected to inputs of the 2N + 1 comparing means components, andfurther comprising digital summing means having inputs thereof connectedto outputs of the reversible counter means components for providing atthe output of the digital summing means the difference between the totalnumber of agreements and the total number of disagreements over theparticular time interval, the plurality of digital correlators reducingthe measurement time required for a desired accuracy of channeldistortion measurement.
 7. The communication channel performancemonitoring system set forth in claim 6 wherein the 2N + 1 signallimiting means components are provided with 2N + 1 different biasvoltages determining 2N + 1 predetermined decision values of the channeloutput signal magnitudes about which 2N + 1 two-level signals aredeveloped.
 8. The communication channel performance monitoring systemset forth in claim 1 and further comprising means for automaticallyvarying a bias voltage level in said signal limiting means inpredetermined steps as a function of the channel output signal tothereby determine a plurality of predetermined decision values of thechannel output signal magnitude about which the signal limiting meanstwo-level output signal is developed, the plurality of decision levelsreducing the measurement time for a desired accuracy of channeldistortion measurement.
 9. The communication channel performancemonitoring system set forth in claim 1 and further comprising countermeans in communication with said second generating means forestablishing a count which is the sum of the total number of agreementsand total number of disagreements and is equal to the number of clockpulses generated over the particular time interval, the crosscorrelation being determined by the count established by said reversiblecounter means divided by the count established by said counter means.10. A method for monitoring the performance of a communication channelby determining the impulse response thereof employing polaritycoincidence correlation for obtaining measurement of cross correlationbetween the channel output signal and a delayed locally generatedpseudorandom noise sequence probe signal and comprising the steps ofgenerating clock pulses and a first pseudorandom noise sequence voltageprobe pulse signal in response to the clock pulses wherein the leadingedges of the probe signal pulses coincide with the clock pulses and theperiods of the probe signal pulses are random integral numbers of aclock pulse period, passing the first probe signal through thecommunication channel being monitored, limiting the signal output of thecommunication channel by developing a two-level signal as a function ofthe relative magnitudes of the channel output signal, generating asecond probe signal corresponding to the first pseudorandom noisesequence signal but with a first variable time delay, comparing thedelayed second probe signal and the two-level limited signal on abit-by-bit basis with respect to the clock pulses, the signal comparisondeveloping an output signal at a first voltage level when the levels ofthe delayed second probe signal and limited signal agree and at a secondvoltage level when the levels disagree, generatiNg a count which is thedifference between the total number of agreements and total number ofdisagreements of the compared delayed probe signal and limited signalover a particular time interval for a first selected delay in the secondprobe signal, and generating a count which is the sum of the totalnumber of agreements and total number of disagreements and is equal tothe number of clock pulses generated over the particular time intervalto thereby determine a first measurement of the cross correlationbetween the signal at the output of the communication channel and thepulse signal by dividing the difference count by the sum count.
 11. Themethod for monitoring the performance of a communication channel setforth in claim 10 and further comprising repeating the steps ofgenerating a first probe signal, limiting the signal at the output ofthe communication channel, generating a second probe signal with avariable time delay, comparing the delayed probe signal and limitedsignal and generating counts which are respectively the differencebetween and sum of the agreements and disagreements over a particulartime interval for a plurality of different variable time delays of thesecond probe signal including zero delay for determining a plurality ofdifferent measurements of the cross correlation between the signal atthe output of the communication channel and the probe signal from whichthe channel impulse response is determined.